CCSD(T) calculation of NMR chemical shifts: consistency of calculated and measured 13C chemical shifts in the 1-cyclopropylcyclopropylidenemethyl cation

The source of conspicuous disagreement between theory and experiment for the chemical shift of Cα in the 1-cyclopropylcyclopropylidenemethyl cation is identified as an inadequate treatment of electron correlation effects in a previous theoretical study. When the sophisticated CCSD(T) method is used, all calculated 13C chemical shifts agree with the experimental values to within 2.2 ppm. These findings undermine the basis of an earlier suggestion that the selectively poor performance of theory for the Cα nucleus can be attributed to environmental effects in which solvent interactions significantly perturb the geometrical structure of the cation.